Steam flood process for producing oil



United States Patent Oflice 3,385,360 STEAM FLOOD PROCESS FOR PRODUCINGOIL Robert V. Smith, Bartlesville, Okla, assignor to Phillips PetroleumCompany, a corporation of Delaware No Drawing. Filed Feb. 1, 1966, Ser.No. 523,882 3 Claims. (Cl. 166-11) ABSTRACT OF THE DISCLOSURE Oil isproduced from an oil stratum by a direct steam drive using normal steaminjection rates in the range of 5,000 to 50,000 pounds per hour for anextended period of at least several weeks and preferably several monthsfollowed by a period of injecting steam at a substantially reduced rateamounting to no more than three fifths of the previous injection rate,and alternating these steps of full injection rate and reduced injectionrate while recovering oil from an offset production well.

This invention relates to a process for producing oil from an oilstratum by steam flood or drive.

Oil is being produced from oil strata by steam flooding, which comprisesinjecting steam thru one or more injection wells and driving the steamthru the oil stratum toward one or more production wells so as todisplace oil into a production well(s). Huff and puff steam operation isalso in use. This procedure involves injecting steam into an oil stratumthru an input-output well with offset wells shut in or otherwise notopen to flow thereinto, so as to build up additional reservoir pressure,and thereafter terminating steam injection and opening the injectionwell to flow so that the heated oil (now less viscous) flows into theinjection well. In the huff and puff operation there is no drive ofsteam from one well to another but rather production from an annular orgenerally circular section of the stratum around a single Well. Indirect drive steam flooding, steam is continuously injected thru theinjection well(s) at least until breakthru in the production well(s)occurs. Thus, the steam requirements are considerable in this type ofoperation. To illustrate, in one field of operation the injection rateof steam is 44,000 pounds per hour.

This invention is concerned with a steam flooding process whichconserves steam by making more effective use of a minimum amount ofinjected steam.

Accordingly, it is an object of the invention to provide an improvedsteam flooding process for the production of oil from an oil stratum.Another object is to provide a direct drive steam process which makesmore effective use of steam and the latent heat therein in a directdrive steam process. A further object is to provide a steam floodingprocess which produces oil more economically than processes heretoforeknown. Other objects of the invention will become apparent to oneskilled in the art upon consideration of the accompanying disclosure.

The invention involves injecting steam thru an injection well into anoil stratum at normal rates for a substantial period of time, so as toform a steam bubble adjacent or surrounding the injection well, andthereafter substantially reducing the rate of steam injection to no morethan that of the initial injection period for another substantialperiod, the production well(s) being open to flow during these periods.The normal steam injection rate during the initial injection period isin the range of 5,000 to 50,000 pounds of steam per hour and usuallythis rate will be in the range of 10,000 to 45,000 pounds per hour. Evenhigher steam injection rates are possible, depending upon the size ofthe input well, the permeability and thickness of the stratum, thecharacter 3,385,300 Patented May 28, 1968 of the in-place oil, etc. Theinitial steam injection is continued until the injected steam is in therange of 0.05 to 0.6 pore volumes of the sweep pattern and, preferably,in the range of 0.1 to 0.25 pore volumes. The initial injection periodis at least several weeks and up to six months or more, depending uponthe character of the stratum, the oil, and. the well pattern or spacingbeing utilized. The initial injection period will usually be in therange of two to six months.

After the initial steam injection period, the steam injection rate iscut back substantially and, preferably, to not more than 7 the initialinjection rate. It is preferred to reduce the steam injection rate to nomore than that of the normal steam injection rate, during the reducedinjection rate period. A substantial advantage is obtained even when theinjection rate is cut substantially to zero. In other words, theinjection of steam may be cut off entirely for a substantial periodbetween normal injection phases of the process. The period of lowersteam injection rates or no steam injection at all ranges from X; toseveral times the initial or normal steam injection rate periods. Thus,in an operation utilizing normal injection rates for a period of sixmonths, the period of reduced or zero injection steam rate is in therange of three weeks to eighteen months or longer.

The cycling of steam in accordance with the invention (a period ofnormal injection rate followed by a period of reduced injection rate) iscontinued at least up to steam breakthru in the production well(s) andmay be continued until oil is no longer produced. It is feasible toinject a slug of gas or a slug of gas followed by a slug of water asdriving agents after sufiicient steam has been injected, intermittently,to provide sufficient steam bubble and heat to sweep the well pattern,thus further conserving steam and heat.

The successive periods of normal steam injection may be substantiallyequal or they may be varied either up or down in length, it being notessential to maintain either the periods or normal injection or ofreduced injection the same in any given field operation.

Steam injection temperatures will range from 350 to 750 F. Thus, thesteam may be either a low-grade steam or a super-heated steam.

In a recent field operation in Nacatoch Sand reservoir of the SmackoverField in Ouachita County, Arkansas, it became necessary to reduce therate of steam injection for several days to approximately the normalrate. Upon resumption of normal operations (injection of steam at a rateof 44,000 pounds per hour), a substantial increase in the rate of oilproduction was noticed. Under the reduced rate of steam injection, thepressure and temperature in the steam body in the reservoir decreasedand, after resumption of normal operation, the pressure and temperaturebegan to return to the values previously observed.

A decrease in pressure in the steam body in an underground reservoircauses a realignment in the fluids with the colder oil and water thathave been displaced by the steam moving into the zone of highertemperature previously occupied by the steam. The reservoir rock servesas a heat exchange agent between the steam and the fluids in thereservoir. Thus, the heat exchange between the steam and oil in thereservoir becomes more efi'icient, which accomplishes the purpose of thesteam flood at less expense. The cold fluids move into the hot steamzone under the influence of pressure and gravity drainage. Anotherbeneflt from the cyclic steam flood is the generation of steam withinthe underground reservoir from the natural and induced water containedin the high temperature steam zone. The decrease in pressure on shut-inor reduced injection rate part of the cycle causes the water a in theliquid state to boil and release large volumes of steam. The generationof steam from the natural and induced water in the reservoir causes theoil to be released from around the grains of the reservoir rock, andfurther assists in the realignment of the fluids in the reservoir.

The invention is a straight steam drive from injector well to producerwell and is independent of how thick the reservoir is, how much is openin the injection well, and whether the steam is injected into the top,middle, or bottom of the stratum being produced. Normally, steam isinjected over the entire exposed cross section of the stratum open tothe injection well. The invention is based upon the fact that the natureof steam permits it to move thru the oil-bearing rock or sand too fastfor sufiicient utilization of all of the heat therein so that byterminating injection or reducing injection rates at spaced intervals,each volume of steam thus injected is permitted to contact the rock orsand for a longer time interval. This enables heat to be conducted intoportions of the reservoir or stratum thru which the steam will not passand, thus, the reservoir is heated more uniformly than in conventionalsteam flooding. Termination of injection also results in a pressure dropwhich causes heated oil to move into the path of the steam andsubsequently be displaced. This oil cannot be recovered by continuousinjection.

The invention is being applied in the North Burbank Unit steam injectionproject in Osage County, Okla., in addition to Smackover Field.Operation in the Smackover Field resulted in additional ultimaterecovery of 7,500 barrels of oil for each cycle.

Certain modifications of the invention will become apparent to thoseskilled in the art and the illustrative details disclosed are not to beconstrued as imposing unnecessary limitations on the invention.

I claim:

1. A process for producing oil from an oil stratum by steam drive froman injection well to an offset production well penetrating said stratumcomprising the steps of:

(1) injecting steam into said stratum at normal injection rates in therange of 5,000 to 50,000 pounds per hour thru said injection well for aperiod of one to six months until 0.05 to 0.6 pore volumes of steam havebeen injected;

(2) thereafter, injecting steam at a substantial rate no more than /5 ofthat in step (1) for a period in the range of one half to several timesthe period of p (3) repeating steps (1) and (2) with said productionwell open to flow so as to displace oil thereinto; and

(4) producing oil from said production well.

2. The process of claim 1 wherein said steam is injected at atemperature in the range of 350 to 750 F.

3. The process of claim 1 wherein the rate of steam injection in step(2) is about /2 that in step ('1).

References Cited UNITED STATES PATENTS 2,584,606 2/1952 Merriam et al.1661'1 2,813,583 ll/1957 Marx et al. 166-40X 3,042,114 7/1962 Willman16640X 3,193,009 7/1965 Wallace et al. 16640X FOREIGN PATENTS 511,7688/1939 Great Britain.

STEPHEN J. NOVOSAD, Primary Examiner.

